What does acrylic 'colour shift' mean?

Acrylic paints usually darken in tone as they dry, making colour matching difficult, so artists have to remember to allow for this when mixing wet colours. This is known as ‘colour shift’ and is due to the binder changing from white to transparent as it dries.

However, Winsor & Newton Professional Acrylic overcomes the colour shift problem, so that the colour remains bright, even when it has dried. The binder used in Professional Acrylic has been developed so that it is translucent when wet and dries clear, and the result is virtually no colour shift. It means that colours can be matched more easily from palette to canvas and a painting will actually look like it will, when finished.